Hot melt pressure sensitive adhesives are a popular option for tape and label applications. There are several different grades of pressure sensitive adhesives including permanent grades, semi-permanent grades, removable grades and freezer grades.
Permanent grade adhesives are utilized where it is desired to permanently adhere one substrate to another. These adhesives typically have high peel adhesion values and are utilized for applications such as mailing labels, bottle labels, case seal tape, tamper evident bag closures and so forth.
Semi-permanent grade or repositionable adhesives allow a substrate to be removed for a short period of time after application. These adhesives are designed so that adhesion builds over time until the bond is permanent. These adhesives have good adhesion at refrigerator temperatures. These grades are used for food label applications, i.e. dairy products, for instance.
Freezer grade adhesives are designed to have good adhesion to substrates to temperatures as low as about -30.degree. C. These adhesives are used for frozen food labels and freezer shelf identification.
Removable grade hot melt pressure sensitive adhesives are designed to allow removability of one substrate from another at any time after application. These adhesives, therefore, do not build significantly in peel values or adhesion over time, or in other words, upon aging of the adhesive bond. These adhesives typically have low peel adhesion values. Typical applications include magazine tipping, credit card attachment, removable store shelf marking for sale items, labels for electronics and appliances and so forth.
Hot melt pressure sensitive adhesives typically comprise a block copolymer, a tackifying resin and a plasticizing oil. The block copolymer provides flexibility, integrity and smooth peel adhesion properties. It also further provides a medium for dissolution or suspension of the tackifying resin and the plasticizing oil. The tackifying resin enhances tack properties and adhesion and reduces viscosity and the plasticizing oil reduces peel values, viscosities, glass transition temperatures and storage modulus (G'), and increases flexibility. The low ultimate peel value and lower peel adhesion of the removable adhesive is achieved through the use of higher polymer and plasticizing oil concentrations and lower tackifying resin concentrations in the adhesive. The low levels of tackifier and high polymer and oil concentrations can lead to several problems.
There are many requirements that must be met in the development of removable grade hot melt pressure sensitive adhesives including low viscosities at application temperatures of less than about 175.degree. C., high heat resistance, good cold temperature flexibility, resistance to cold flow, good adhesion to the base substrate, high resistance to bleeding or staining of substrates, good die-cuttability and good quick tack or quick stick. The difficulty in formulating such adhesives is achieving a good balance of desirable properties without sacrificing one property to another. For instance, high heat resistance may be achieved at the expense of good cold temperature flexibility, lower viscosity may be achieved at the expense of stain resistance due to either higher levels of the liquid components or lower levels of polymer, good cold temperature flexibility may be achieved at the expense of resistance to what is known as "cold flow" at room temperature and adhesion and quick tack may be sacrificed to obtain lower, smoother peel characteristics to mention only a few examples.
One of the main concerns in the development of removable grade hot melt pressure sensitive adhesives is poor adhesion to the substrate to which the adhesive is applied in its molten state, or in other words the "base substrate." This phenomenon is referred to in the industry as poor anchorage. This is more of a problem with removable grades because the level of tackifying resin must be relatively low in order to retain removability but because the tackifying resin is the primary adhesion promoter, it can also lead to poor anchorage. The adhesive may detach from the base substrate prior to application of the substrate to another surface or there may be poor bonding of the base substrate to a second surface.
Another problem often encountered is poor stain resistance. Block copolymers act as a means or vehicle for solvating or in other words suspending both liquid components and solid tackifying resins. Block copolymers may have many configurations including A-B-A triblock, A-B diblock, radial and Y-block and A-B-A-B-A-B multiblock configurations. The A block is most commonly styrene and the B block may be ethylene/butylene, ethylene/propylene, isoprene or butadiene. The liquid components may associate with either the A block, often referred to as the endblock, or the B block, often referred to as the midblock, depending on the chemical formula and compatibility. Plasticizers such as oils and midblock associating liquid resins are often used to lower the viscosity, increase flexibility, lower the glass transition temperature and increase the storage modulus or G'. They are also used in removable grades to lower the ultimate peel value. High levels of liquid components relative to the level of polymer may lead to poor stain resistance, however.
Removability can also be accomplished through the use of higher polymer contents but this leads to high viscosities and products which cannot be applied to substrates easily. It can also be achieved through the use of higher molecular weight liquid components such as liquid elastomers but this also leads to higher viscosities. The use of plasticizing oils is often preferable for these reasons. However, high levels of oil can cause staining due to the fact that the polymer can only solvate so much oil before the oil begins to migrate. This migration leads to substrate staining and deterioration of adhesive performance upon aging of the adhesive bond.
A third problem encountered is a build in peel adhesion over time which can ultimately destroy the removability causing undesirable substrate failure and adhesive residue on surfaces to which the base substrate is applied.
There is a continuing need in the art to achieve removable grade hot melt pressure sensitive adhesives which retain an excellent balance of properties. There has especially remained a need in the development of removable grade hot melt pressure sensitive adhesives to have adhesion to base substrates and excellent quick tack while retaining low ultimate peel adhesion, low viscosity and good stain resistance. Good adhesion is typically achieved through the use of high levels of tackifying resin which also reduces the ability to remove the base substrate from other surfaces at a much later date. However, reducing the tackifying resin also reduces the initial adhesion or anchorage to the base substrate. There has been a constant effort in the pressure sensitive adhesive industry to achieve a product with excellent initial adhesion to the base substrate when applied in the molten state, and yet allow easy removability of the adhesive from a surface to which it is applied, by hand pressure for instance, once it is cool.
The present inventors have found hot melt pressure sensitive adhesive compositions which have excellent anchorage to the base substrate, excellent quick tack and excellent stain resistance while retaining a low viscosity and low ultimate peel adhesion therefore achieving a superior balance of properties.